Metadata Caching

[hannahhoward]

What

Once ipfs/go-graphsync#375 is implemented, and once feedback is collected on ipld/go-ipld-prime#395, we can write an initial cache.

Scope

For the first version, we should endeavor to keep things as simple as possible.

• back with go-datastore, unless it's easier to just use something else (i.e. go-storethehash)
• just write a key value mapping of CID + dag-cbor serialized selector (maybe dag-json to satisfy the go-datastore interface?) to dag-cbor encoded metadata list